1. Field of the Invention
This invention relates in general to whipstocks for drilling sidetrack boreholes from a wellbore, and in particular to retrievable whipstocks for use in cased wellbores to cut a window laterally through a casing for passing a drillstring to drill a sidetrack borehole.
2. Description of the Prior Art
Prior art whipstocks have been used for drilling sidetrack boreholes from cased wellbores. A prior art whipstock is typically run into a wellbore as part of a whipstock assembly which includes an anchor means for setting at a well depth to support the whipstock within a casing. Several trips into a wellbore are usually required for cutting a window laterally through a side wall of the casing. Once the window is cut laterally through the casing, a drillstring can then be run through the window to drill a sidetrack borehole.
Prior art whipstocks are typically not retrievable with conventional fishing tools, such as conventional spears and overshot tools. Rather, specialized fishing tools are used which can not transmit as much force to the whipstock as can be transmitted with conventional fishing tools. Specialized fishing tools are generally required since typically only the top of the tapered portion of a prior art whipstock is available for latching onto with a fishing tool.
For a whipstock to be retrievable with a conventional overshot tool, the whipstock should be formed with a larger taper, or face angle, than conventional whipstocks. The face angle of a whipstock is the angle between the deflection surface, that is the whipstock face, and the interior surface of the casing. A larger face angle reduces the longitudinal length of the tapered section of whipstock, which provides an upper portion of the tapered section which extends farther about an interior circumference of the casing. A tapered section which extends circumferentially farther about an interior of a casing is easier to latch onto with a conventional overshot tool.
Although a whipstock tapered section having a larger face angle is easier to latch into a conventional fishing tool, a problem arises in that the tapered section does not extend far enough in a longitudinal direction within the casing. This larger face angle and shorter whipstock tapered section results in reducing the longitudinal length of the window which can be cut in the casing with a particular milling tool. If a window does not extend far enough in a longitudinal direction along the casing, then larger diameter and stiffer drillstrings can not be run through the window and into the sidetrack borehole as could be run if the window extended farther in the longitudinal direction.
Prior art whipstock assemblies have only a single deflection surface for cutting a particular window laterally through a casing. This restricts operators to a deflection surface having only a particular face angle. In particular, prior art whipstock assemblies do not include multiple whipstocks for drilling a singular window laterally through a casing.
Milling tools are lowered into wells for engaging with a whipstock surface to cut a window through casing. Prior art full gauge mills can not be run to mill a full gauged window through the casing on a singular trip, but rather are run on subsequent trips after a starting mill is run. As used herein, a full gauge window is a window which is milled using a full gauge milling tool, which is herein defined to be a milling tool having a maximum exterior diameter which is substantially the largest diameter which can be passed interiorly within the casing and still have adequate clearance with the internal casing diameter for tripping within the cased wellbore. An under gauged milling tool is herein defined as a milling tool having a maximum exterior diameter which is significantly smaller than the largest diameter which can be passed interiorly within the casing with adequate clearance for tripping in and out of the wall.
Further, prior art whipstocks typically provide a deflection surface, or whipstock face, having only a singular face angle which extends to an outer diameter of the whipstock. This can result in a section of casing being left adjacent to the downhole portion of the whipstock face after the window is cut. The lower portion of the whipstock and the adjacent section of casing form a space which can trap debris, such as cuttings from the milling operation and other wellbore debris. The deflection surface can then press debris into the casing to wedge the debris between the casing and the whipstock as the whipstock is urged to move uphole.
The section of casing can be left adjacent to the lower end of the whipstock face after cutting a window for two reasons. First, as a window is cut laterally through a casing, the mill can lift off of the deflection surface prior to completing the window and leave a section of the casing adjacent to the lower end of the whipstock face. Second, a milling tool is operated to cut a window by rotating to the right, which is viewed as rotation in a clockwise direction when looking in a downhole direction. As the milling tool is rotated to the right, it will usually walk off of the lower end of the whipstock face in a path which extends in a right hand spiral as the milling tool exits the window, which also leaves a small section of casing adjacent to the lower end of the whipstock face.
As the whipstock is urged to move upwards within a wellbore, the deflection surface is at a face angle to the section of casing. This face angle results in a lateral force component being passed from the deflection surface and to the debris, which presses the debris between the deflection surface and the section of casing. The debris can then become wedged between the whipstock and the casing to stick the whipstock within the casing and prevent removal of the whipstock from the wellbore.
Referring to FIG. 1, a longitudinal section view of a wellbore depicts prior art whipstock 11 within casing 13, through which a mill has cut a window 15 along path 17. As the mill passed along path 17 to cut window 15, the mill lifted off of whipstock 11 to leave a segment 19 of casing 13. Space 21 between segment 19 of casing 13 and whipstock 11 acts as a trap for catching debris 23.
With reference to FIG. 2, a side view of casing 13 and whipstock 11 of FIG. 1 depicts window 15. The edges of deflection surface 25 of whipstock 11 are shown as hidden lines to illustrate how a mill typically walks to the right as it cuts the lower portion of window 15 through casing 13. A mill walking to the right leaves segment 27 of casing 13 adjacent to deflection surface 25 of whipstock 11, even if the mill does not lift off of deflection surface 25 of whipstock 11 prematurely to leave casing segment 19, as shown in FIG. 1.
Referring to both FIG. 1 and FIG. 2, debris 23 can then become trapped within space 21 between deflection surface 25 and adjacent segment 27. Additionally, other debris may become lodged between deflection surface 25 of whipstock 11 and an interior surface of casing 13 as whipstock 11 is moved uphole, besides debris 23 which is trapped in space 21 between whipstock 11 and casing 13 as window 15 is milled, or as the sidetrack borehole is drilled.
When whipstock 11 is urged to move uphole, deflection surface 25 of whipstock 11 urges debris 23 laterally into casing 13 with a lateral force component which arises from deflection surface 25 being disposed at a face angle to an adjacent interior surface of casing 13. In particular, when whipstock 11 is urged to move uphole within casing 13, deflection surface 25 can apply a force to debris which is adjacent to deflection surface 25. This applied force can have a general direction which is normal to the face of deflection surface 25. The force will then have a force component which is in a general direction that is normal to the interior surface of casing 13, that is, which presses the collected debris laterally into the interior surface of casing 13.